I think overall and it's unlikely seeing as it's a handheld, I would love a GBA classic. Another one would be the Gamecube classic/mini. An original DS classic would be fantastic too and handheld classics in general are needed seeing as the new Nintendo Online service doesn't offer all that much yet.
(I own a few DS games but a lot of them are secondhand copies from Ebay and not to my surprise are in pretty mediocre shape.) I would love to own a GBA again but finding boxed games are pretty rare to come by in my opinion. Expensive too.
Ah if only I kept my Wii U. Virtual Console was something I regret not experiencing even not being able to on the original Wii!
PS2 classic would be cool too but probably limited seeing as the PS1 classic was. Who knows. :)